Consider this: option to enable displaying promotion post can be buried in the Settings - with additional information, that enabling this help support Steem network.

You can ask a user once, whether they want to consider enabling that, but only if:

• you will detect, that certain user is very active, and spends X time a day on your website (probably such user already know everything about Steem)
• or easier approach. Ask only users, which has... I don't know... maybe 10 000 STEEM or more.

I will emphasize this one more time. This can be disabled by default :)